It’s been a while, but I have something to add to this question.
I just received a new horseshoe magnet. It came with a small piece of steel that’s big enough to span across the two poles. The piece of metal is called a keeper. Wikipedia says the purpose of the keeper is to preserve the strength of the magnet by completing the circuit and keeping the magnetic domains aligned.
When I dip the horseshoe magnet into filings, it’s covered in filings stuck to the poles. When I dip the horseshoe magnet with the keeper in place into the filings, very few filings stick to the magnet. None stick to the (ferromagnetic) keeper. The only filings that stuck to the magnet are along the edges of the poles where they touch the keeper.
So apparently there is something to the idea of the magnetic field being blocked by short circuiting. Perhaps a tin can just doesn’t have enough oomph to produce the effect.
I never thought of magnetic fields as having a circuit, much less being able to short circuit a magnetic field, so this is all very interesting.